Semiconductor wet chemical processing in-line heaters are known in the art. A conventional wet chemical processing in-line heater can be either a radiation type or a direct-heating type. A radiation-type heater often includes a Teflon tube for carrying wet chemicals to be heated and an infrared heating source, such as an infrared lamp, located outside the Teflon tube, wherein the wet chemicals are heated by the infrared lamp through radiation. Unfortunately, radiation-type heaters are often inefficient at heating and are prone to contamination. It is well known that heating by radiation inevitably causes a significant amount of heat loss. Moreover, Teflon tubes are vulnerable to chemicals at high temperatures and are likely to release particulate matters into the chemicals to be heated, thereby leading to contamination problems.
A conventional direct-heating type heater includes a tube with a resistive heating element on the exterior surface of the tube, which is electrically connected to a power source. When power is supplied, the resistive heating element heats the chemicals flowing within the tube by conductive heating, which is more efficient than heating by radiation since there is less heat loss. Among the conventional materials, quartz has been known as a superior material for the tube for direct-heating type heaters because quartz has better chemical corrosion resistance properties than any other conventional materials such as stainless steel and can withstand deionized water and corrosive chemicals, such as acids. However, the corrosion resistance properties of quartz degrades as the temperature increases. Therefore, like other conventional materials, quartz is also vulnerable to chemical corrosion at high temperature, as evidenced by corrosion often occurring at portions of a quartz tube where the tube contacts the resistive heating element. For example, the quartz tube can be etched by corrosive chemicals to a depth of about 0.040 inches (0.016 cm) after about two (2) years of use. Like other conventional materials, quartz cannot carry hydrofluoride (HF) solution due to its inherent vulnerability to HF solution.
While quartz exhibits superior chemical resistance properties than other conventional materials, quartz has a deficiency in that it is difficult to find an appropriate dielectric material as a protective layer to cover over the tube and the resistive heating element without causing cracks between the protective layer and the substrate. Quartz has a relatively low coefficient of thermal expansion (CTE) of about 5.5×10−7/° C., while the commonly used dielectric materials, such as silicate based glass frit, have much higher CTEs ranging from about 4.5×10−6 to about 9×10−6/° C. The difference in thermal expansion among the protective layer, resistor element, and the substrate creates thermal stresses, which results in cracks on the substrate. The general solution to this problem is to provide a tube without a protective layer. In the absence of a protective layer, however, the resistive heating element is exposed to the outside environment, which could lead to damage of the resistive heating element.
Conventional tube materials pose another problem in that a reliable electrical connection between the resistive heating element and the power source is often difficult to achieve. In some known heaters, lead wires cannot be used to directly connect the resistive heating element to the power source due to the difference in thermal expansion between the metallic lead wires and the tube. A common solution to this problem is to use a mechanical device, such as a loaded spring, to connect the resistive heating element to the lead wires. However, fatigue of such a mechanical device can occur after long periods of use, which may significantly increase the probability of failure of the connection between the resistive heating element and the power source. Moreover, fatigue is accelerated when the mechanical device is used at higher temperatures.